The epidermal growth factor receptor (EGFR) is a ligand activated receptor tyrosine kinase and a member of the ErbB receptor family. EGFR ligands include members the epidermal growth factor (EGF) family such as EGF, transforming growth factor-alpha (TGFα), heparin binding EGF-like growth factor (HB-EGF), amphiregulin (AR), epiregulin (EPR), betacellulin (BTC), epigen and neuregulin (NRG)-1, NRG-2, NRG-3 and NRG-4. EGFR ligand dysregulation is apparent in a number of diseases. For example, in non-small-cell lung cancer, increased plasma TGFα is associated with erlotinib resistance and increased amphiregulin is an indicator of poor prognosis.
EGFR is a target for anti-cancer therapies as it is over expressed in a large number of cancers. For example, more than 80% of all head and neck cancers (HNCs) overexpress EGFR. Signalling from EGFR results in activation of downstream phosphoinositide 3-kinase (PI3K)/Akt and Ras/Raf/MAPK pathways that promote tumour proliferation, invasion, metastasis, angiogenesis and apoptosis inhibition which all contribute to cancer progression and poor patient prognosis. A number of inhibitors of EGFR, acting as tyrosine kinase inhibitors, have been developed as anti-cancer therapeutics. However, limited results have been achieved in clinical trials with tyrosine kinase inhibitors targeting EGFR, including gefitinib and erlotinib and the monoclonal antibody cetuximab, in a range of cancers including HNC. One of the major challenges facing the clinical use of anti-EGFR tyrosine kinase inhibitors is the inherent and acquired resistance of cancers to these therapeutics. There is increasing interest in, and a growing need for, the development of effective approaches to overcome tyrosine kinase inhibitor resistance and to increasing the efficacy more generally of tyrosine kinase inhibitors.
microRNAs (miRNAs) are an abundant class of highly conserved, small (typically 21-25 nucleotides) endogenous non-protein-coding RNAs that negatively regulate gene expression. miRNAs bind specific 3′-untranslated regions (3′-UTRs) within messenger RNAs (mRNA) to induce mRNA cleavage or translational repression. Individual miRNA typically bind incompletely to their cognate target messenger RNA (mRNA) and a unique miRNA may regulate the expression of multiple genes.
miRNAs are generated from RNA precursors (pri-miRNAs) that usually contain several hundred nucleotides transcribed from regions of non-coding DNA. Pri-miRNAs are processed in the nucleus by RNase III endonuclease to form stem-loop precursors (pre-miRNAs) of approximately 70 nucleotides. Pre-miRNAs are actively transported into the cytoplasm where they are further processed into short RNA duplexes, typically of 21-23 bp. The functional miRNA strand dissociates from its complementary non-functional strand and locates within, the RNA-induced-silencing-complex (RISC). (Alternatively, RISC can directly load pre-miRNA hairpin structures.) miRNAs bind the 3′UTRs of target mRNAs and important in this binding is a so-called ‘seed’ region of approximately 6-7 nucleotides near the 5′ end of the miRNA (typically nucleotide positions 2 to 8). The role of the 3′ end is less clear. miRNA-induced regulation of gene expression is typically achieved by translational repression, either degrading proteins as they emerge from ribosomes or ‘freezing’ ribosomes, and/or promoting the movement of target mRNAs into sites of RNA destruction.
miRNAs are crucial to many normal cellular functions and are involved in processes such as stem cell division, embryonic development, cellular differentiation, inflammation and immunity. Increasingly, specific miRNAs, and expression patterns and altered regulation of expression of individual miRNAs, are also being implicated in a variety of disease conditions, including cancer. Some miRNAs are altered in cancer and may act as tumour suppressors or oncogenes. For example, let-7d (a member of the let-7 family of miRNAs) regulates RAS oncogene expression in normal head and neck tissue although let-7d expression is reduced in many head and neck cancers causing upregulation of RAS expression, increased tumour growth and reduced patient survival. In contrast, miR-184 expression is upregulated in tongue squamous cell carcinoma, leading to increased expression of the oncogene c-Myc, increased cell proliferation and tumour growth.